1. Field of the Invention
The present invention relates in general to digital subtraction fluorography, and more particularly concerns a method and apparatus for use in connection with temporal subtraction of X-ray images with phase information.
2. Discussion of the Background
Digital subtraction fluorographic apparata and methods have previously been used for visualizing the flow of an X-ray contrast medium through blood vessels. One digital fluorography technique involves projecting an X-ray beam through a body, converting the resulting X-ray image to an optical image with an image intensifier, converting the optical image to analog video signals with a video camera and then digitizing the video signals to form a matrix of digital values that correspond in magnitude to the intensity of the picture elements (pixels) that compose the image. In temporal imaging, an image of a region of the anatomy that contains the blood vessels of interest is obtained before an intravenously injected X-ray opaque medium reaches the vessels. This image is typically stored as a mask image. When the X-ray contrast medium begins to flow through the vessels, a series of live images are obtained. The mask image is then subtracted from the successive live images to produce a sequence of difference images. The object of the subtraction is to cancel all unchanged image content such as from bone and soft tissue in the mask and live images so that only the image of the contrast medium containing blood vessels remains for display.
In the art known to applicant, the data for one or both or even more images in succession are usually weighted or otherwise operated on to bring about cancellation of obscuring background and to have the anatomy of interest remain. However, such techniques neither recognize nor take into account that the concentration of contrast medium in the vessel as a function of time. Phase information contained in the difference image at any moment in time is ignored, and not utilized for diagnosis purposes.
One method assuming that the concentration of contrast medium in the blood vessels varies with time is to plot concentration for each pixel in the selected region of interest versus time. A plot of concentration versus time results in a curve representing the blood flow in the blood vessels of interest. However, the prior method is characterized by the following disadvantages:
(a) it is impossible to provide phase information contained in the blood flow over the whole of the blood vessels of interest;
(b) it is impossible to measure the concentration of contrast medium which varies with time, in the selected portion of the blood vessels of interest; and
(c) it is impossible to superimpose the outline of the blood vessels defined by the blood flow containing the X-ray contrast medium and the phase information arising from the blood flow in the blood vessels.
In other words, an X-ray contrast medium is not being observed as it flows through the blood vessels, and the only contrast difference between successive X-ray images, which is due to the leading edge of the contrast medium having advanced during the integrating interval, is displayed.